Polyethylene thin films as disclosed in WO 2007/015416, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2011-516624A, and Japanese Unexamined Patent Application Publication No. 2001-240690A, especially polyethylene microporous films, deform microscopically at room temperature because the glass transition temperature of the resin itself is below room temperature. For this reason, it is difficult to uniformly obtain the physical properties required when used as the separator of a lithium ion battery or the like. In particular, when the film is large in the width or longitudinal directions, it is extremely difficult to assure uniformity of various physical properties across the entire face.
Thus, in the past, only the portions of a film where the various physical properties were uniform were selected to be used in various applications.
In the development of lithium ion batteries for modern automobiles and consumer electronics, as battery size has increased and as produced quantities have increased, it has become necessary to enlarge the area in both the width and longitudinal directions of the polyethylene microporous film used as the separator. Thus, it has become greatly desired to examine the causes of non-uniformity of films and to develop a polyethylene microporous film having excellent uniformity of various characteristics and planarity in a film that is large in the width and longitudinal directions.
Thus, it could be helpful to provide a polyethylene microporous film having excellent planarity and uniformity of the various characteristics required in the separator of a battery, even in large-area films.